Add proportional analysis for affected watersheds

Request from a Geomorphologist: automatically evaluates the proportion of vegetation burn severity in all named catchments impacted by the fire, and summarizes the proportions of named watersheds affected, along with the proportion of moderate, high, and Moderate+high severity

Their process:
take the watershed area and then examine how much of the watershed burned for a proportion of watershed area affected (%) and # of hectares. Add up the moderate and high burn severity (ha) and divide by watershed are to get a proportion burned at elevated severity. They this number to inform their hazard likelihood rating.

No files or tests are named; start by locating the existing watershed/catchment and burn-severity analysis entry points. Use the stated watershed-area and moderate/high severity calculations as acceptance checks, including hectares and percentages for each named impacted catchment.

Written by the indexing model from the issue text.